1. Field of the Invention
The present invention relates generally to hydraulic control systems for automatic automotive transmissions and more specifically to a system suited to controlling the operation of a four speed type transmission.
2. Description of the Prior Art
JP-A-58-156757 discloses a hydraulic control system for a four speed type automatic automotive type transmission which includes a plurality of shift valves. These valves are basically responsive to throttle pressure indicative of the load on the engine and governor pressure which is indicative of the vehicle speed. By appropriately arranging these valves it is possible to derive a suitable 1-2-3-4 shift pattern.
However, with this arrangement when the vehicle slows to a stop, backlash noise tends to be produced. That it is to say, at low vehicle speeds when a 3-2-1 downshift sequence take place, or when, at relatively high vehicle speeds, a 3-2 downshift occurs and a load is placed on the torque outputted by the output shaft of the transmission (i.e. engine braking wherein the engine is driven by the wheels of the slowing vehicle), a momentary reversal of torque occurs. As a result, the gears which are meshing undergo a momentary torque fluctuation and produce the backlash noise.
To overcome this problem it is possible to induce a 3-3 downshift at low vehicle speeds. In order to achieve this the 3-2 downshift line and the 2-1 downshift line should intersect under low throttle valve settings. However, in the event that the 2-3 downshift line is set in the low vehicle speed range, especially at low throttle settings, the 2-3 upshift line is also drawn into this region.
Accordingly, under relatively low engine load (viz., small throttle valve settings) a 2-3 upshift tends to occur very shortly after the transmission has undergone a 1-2 shift and deteriorates the shift feeling.